This invention relates to communication systems. More particularly, and not by way of limitation, the invention is directed to a system and method of performing cell measurements in a telecommunications system.
In the forthcoming evolution of the mobile cellular standards, such as Global System for Mobile Communications (GSM) and Wideband-Code Division Multiple Access (WCDMA), new modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration of the old cellular systems to the new high capacity high data rate system in the existing radio spectrum, the new system has to be able to operate on a flexible bandwidth (BW). A proposal for such a new flexible cellular system is 3G Long Term Evolution (3G LTE), which is an evolution from the 3G WCDMA standard. This system utilizes OFDM as a multiple access technique (called OFDMA) in the downlink and is able to operate on a bandwidth spanning from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be possible in this high bandwidth system.
The 3G LTE system allows for use in a “Reuse One” fashion (i.e., all cells share the same carrier frequency). Therefore, neighbor cell measurements needed for mobility (handover) purposes can be made in similar fashion as in WCDMA. Furthermore, the different BW possibilities in LTE introduces additional neighbor (NB) cell measurements that need to be considered. For example, in some scenarios there may be a “hot spot” cell with a specific bandwidth (e.g., 20 MHz), while neighboring cells may be using another bandwidth (e.g., 5 or 10 MHz). Similar scenarios may occur in the border between countries or other geographical or political boundaries.
There are several scenarios where different NB cell configurations need to be handled in LTE. Intra-frequency neighbor cell measurements are performed by a user equipment (UE) when the current and target cell operates on the same carrier frequency. In this case, the UE is able to carry out such measurements without measurement gaps. Neighbor cell measurements performed by the UE are considered as inter-frequency measurements when the neighbor cell operates on a different carrier frequency in comparison to the current cell. In this situation, the UE is unable to carry out such measurements without measurement gaps.
Depending on whether the UE needs transmission/reception gaps to perform the relevant measurements, measurements are classified as gap assisted or non-gap assisted. A non-gap assisted measurement is a measurement on a cell that does not require transmission/reception gaps to allow the measurement to be performed. A gap assisted measurement is a measurement on a cell that requires transmission/reception gaps to allow the measurement to be performed. Whether a measurement is non-gap assisted or gap assisted depends on the current operating frequency. The UE determines whether a particular cell measurement needs to be performed in a transmission/reception gap.
In the situation where cells operate on the same carrier frequency, gaps are not needed to perform the measurements. If the cells' carrier frequencies differ, gap assisted measurements are needed which are independent of the UE/cell bandwidth. These measurement gaps are provided and controlled by the network.
FIG. 1A is a simplified block diagram illustrating an intra-frequency measurement scenario in LTE. In FIG. 1, a UE 10 communicates with a current cell 12. The current cell 12 and a target cell 14 have the same carrier frequency and bandwidth. This is the most common measurement scenario. In this scenario, measurement gaps are not required.
FIG. 1B is a simplified block diagram illustrating a second intra-frequency measurement scenario in LTE. The current cell 12 and the target cell 14 have the same carrier frequency. However, the bandwidth of the target cell is less than the bandwidth of the current cell.
FIG. 1C is a simplified block diagram illustrating a third intra-frequency measurement scenario in LTE. In this scenario, the current cell 12 and the target cell 14 have the same carrier frequency. However, the bandwidth of the target cell is greater than the bandwidth of the current cell.
FIG. 2A is a simplified block diagram illustrating a first inter-frequency measurement scenario in LTE. The current cell and the target cell have different carrier frequencies. In addition, the bandwidth of the target cell is less than the bandwidth of the current cell and the center part of the bandwidth of the target cell is within the bandwidth of the current cell. In this scenario, since it is an inter-frequency scenario, measurement gaps are utilized.
FIG. 2B is a simplified block diagram illustrating a second inter-frequency measurement scenario in LTE. The current cell and the target cell have different carrier frequencies. In addition, the bandwidth of the target cell is greater than the bandwidth of the current cell and the center part of the bandwidth of the target cell is within the bandwidth of the current cell.
FIG. 2C is a simplified block diagram illustrating a third inter-frequency measurement scenario in LTE. The center part of the bandwidth of the target cell is outside the bandwidth of the current cell. In this scenario, measurement gaps are required.
The above figures show the different NB cell configurations that are encountered and require action in LTE. For handoff measurements, typically only a subfraction of the entire bandwidth is used for cell search (i.e., 1.25 MHz). Cell measurements are denoted in FIGS. 1 and 2 as measurement bandwidth (Meas BW). In FIG. 1A, this is the most common scenario corresponding to legacy intra-frequency measurements. In LTE, the scenarios of FIGS. 1B and 1C are also defined as intra-frequency measurements. Since the carrier frequency for the NB cells in FIGS. 1B and 1C is the same as the serving cell, the UE typically performs the measurements on these NB cells without interruption (i.e., no measurement gap) in the data reception. In FIG. 2C, a pure (legacy) inter-frequency measurement scenario is shown (i.e., similar to the WCDMA case). For this scenario, a gap in the reception from the serving cell is needed to conduct the measurements, such that the radio can be retunded to the NB cell carrier frequency. For LTE, in FIGS. 2A and 2B, the scenarios also are inter-frequency measurements and reception gaps are required. In these scenarios, the carrier frequency for the NB cells is not aligned with the carrier frequency for the serving cell. However, the reception bandwidth for the UE still covers the measurements portions in FIGS. 2A and 2B, without changing the local oscillator frequency.
A system and method of performing cell measurements for all the scenarios discussed above is needed. Currently, there are three different existing solutions for performing the measurements on all the scenarios. First, gaps may be created in the intra-frequency measurement scenarios. When the UE is close to the cell border, the UE requests an interruption in the reception in order to allocate the Fast Fourier Transforms (FFT) to the neighbor cell. Thus, a similar way is needed for inter-frequency handoff. The disadvantage with this solution is that a lower throughput is achieved due to the need for interrupting the data reception.
In the second existing solution, synchronized base stations are utilized. To accomplish this solution, all cells have the same timing when using the FFT. All cells' (both serving and neighbor) pilot signals are detected and the signal strength is estimated. The disadvantage of this solution is that the cells must be synchronized.
In the third existing solution, two FFTs are utilized. One of the FFTs is used for serving cell detection and one of the FFT is used for neighbor cell measurements. Since two FFTs are used, an increased chip area cost in the UE is necessary.
Therefore, a system and method for performing the cell measurements utilizing a single FFT in both the inter-frequency and intra-frequency scenarios is needed. The present invention provides such a system and method.